As the electrothermal products are increasingly popular in the fields of household appliances, electrical equipment, electronic aid, motor industry, etc, the application of the electrothermal element is becoming wider and wider. Therefore, the requirement of the performance of every aspect of the electro thermal element is getting higher and higher.
The electrothermal element (mainly nickel chrome) of the prior art has the following disadvantages: Small power density, normally about 15 watt/cm2, which cannot satisfy the need of a high thermal-shock and a low resistance voltage; low toggle speed, most of which have thermal inertia; small echanical strength, low vibration resistance and low thermal-shock; relatively big volume of the element, which occupies larger space and is not easy to install; difficulty in carrying out the coplanar and curved-surface design between the controlling-temperature material and the electrothermal material; inadequate temperature-controlling effect and energy-saving effect; short life-span; side effect, which does harm to the health; low inoxidability; and bad manufacturability.
The problems with the traditional PTC (i.e. the abbreviation of Positive Temperature Coefficient) heater are as follows.
The traditional PTC heater connects several disk or honeycomb-fashion PTC elements and a radiator through splicing and holding, which generates an electrode and forms the subassembly of a heater. The connected PTC heating element subassembly is adhesive connected by silica gel with the corrugated neat sink strips. This structure causes unsteady transient output, easily becomes aged, crocks up transient output critically and noumenon has electrification, which possibly gives rise to short circuit or even fire.
Mechanical holding, instead of adhesive connecting, and integrally finned radiation can improve the radiating effect, which overcomes safety problems with the adhesive PTC, e.g. tackless, aged, noumenon electrification. But the surface temperatures of PTC electrothermal element are mainly at about 250° C., whereas, the leading of holding and electrode is utter significant. Besides poor contact, short circuit and unevenly distribution of electric field, the puncturing caused by partially overheat, owing to the mechanical poor contact, must be avoided.
When several elements are series connected, the distribution of the voltage changes promptly, owing to the differences of the characteristics and temperature—rising of the elements, which gives rise to a vicious circle. The elements, which have high voltage drop and rising temperature, are likely to break down and have a chain reaction. Therefore, the election of the elements is fairly important in series connection.
When several elements are parallel connected, the power of the system will increase within its limitation. But after the confirmation of the system, the radiating power of several elements does not amount to the total radiating power of every element. Actually, it is much less than the total. In parallel connection, there will be a strong impact current several minutes after electrification. Owing to the discreteness of the elements, the rate of the temperature rising of each element varies. As a consequence, an impact current does not amount to the total number of each impact current, but is less than the total number. Nonetheless, the element with less impact current should be elected in parallel connection.
According to the international requirement, the reliable experiment of the PTC heating element is that in the windless situation, the voltage attenuation is less than 8% when running 1,000 hours in the voltage rating. In reality, most of the voltage attenuations of the products are more than 15%. Owing to the problems above, the appliance of the series connected or parallel connected PTC heater is limited on a large scale.
In addition, the heating element of a current thick film circuit is a resistance element, which is lack of temperature controllability. The applied substrate falls into two categories. The first one is a high-temperature cofired ceramics electrothermal element. It is prepared though directly printing the resistance paste on the greenpressing of the alumina ceramics, laminating and batching out; then jointly burning at about 1,600° C. The new mid- and low-temperature heating element is a brand new product, following the heating alloy, the galvanothermy tube and the PTC heater element. This kind of heater element with 96 AI2O3 ceramics substrate is a new product. But the common demerits of the ceramics substrate are high brittleness, low mechanical shock resistance and thermal-shock resistance and the outer size can merely be length≦120 mm width≦100 mm thickness 0.5-1.5. Owing to the art limitation, the sizes of current products are merely 70×20, 70×15, 70×10. The heat surface power in real test is ≦25 w/cm2. Owing to the size limitation of the ceramics substrate and the bad machining property, the product cannot be printed, incised and installed on a large scale. The usage of ceramics substrate is so unitary that it cannot overcome complicated problems such as electromagnetic interference. Therefore, there is so extensive a limitation to the range of appliance that it cannot lead the trend and be widely utilized The other one is 430 stainless steel (National Standard Number 1Cr17) substrate. At present, only Dupont, Est in the U.S and Heraeus in Germany and a few other companies have already mastered the serial paste manufactory art, whose thick film circuit substrate is American Trademark 430 (stainless steel 1Cr17). Furthermore, the said serial paste does not relate to the PTC electronic paste. The made electrothermal element is resistance electrothermal element, therefore, the electrothermal element does not have temperature controllability.
Because 1Cr17 is only an ordinary pure high ferritic stainless steel, whose quality coefficient of carbon is about 0.1% and which contains a small quantity of nitrogen. 1Cr17 is much larger compared with austenitic stainless steel. The demerits of 1Cr17 are delicate material, bad mechanical processing technique and welding procedure, which largely limit its appliance. Austenitic 1Cr18Ni9 serial stainless steel, however, has high plasticity and toughness. Therefore, an Austenitic 1Cr18Ni9 serial stainless steel has a good kinking, curling and punching forming, and a good mechanical processability so that it can be made into components, containers or pipelines of different shapes. Therefore, it is inevitable that a controllable electrotermal element of a thick film circuit with 1Cr18Ni9 serial stainless steel substrate is the trend of the art development.